You do not need SLI to run multiple nvidia GPUs for Folding@Home. Each card only needs to be seen as a video card adapter. There isn't much traffic, so a 4x PCI Express lane isn't a bottleneck either. Vista is more efficient than XP for the Nvidia GPU client. In XP, 1 GPU uses 1 core. In Vista, 1 GPU uses ~ 20% CPU. However for Vista, the card needs to be connected to a monitor or at least have a terminator to be detected.

Some 790FX mobos are good for folding since they contain 4 PCI-e slots. You can run a few lower-powered GPU like the 8800GT and finish lots of WUs.